Polymeric materials have many attractive properties, but their mechanical properties are insufficient for many structural applications. Fibre reinforced composites, wherein approximately 6 to 25 μm diameter fibres having high stiffness and/or strength, typically carbon, glass or aramide fibres, are embedded in a polymeric matrix have substantially higher mechanical properties allowing to reach very advantageous mechanical properties to density ratios. The reinforcing fibres may be short, long or continuous. In the latter case, the continuous fibres can be oriented differently depending on the stress field a particular part is to be submitted to. The reinforcing fibres may be arranged in a fabric form, such as weaves of different patterns, braids, or knits, or they can be laid in a mould or filament wound following a predetermined pattern.
When a fibre reinforced composite is submitted to a stress field, the stress is transferred from the matrix to the fibres through the matrix-fibre interface. If the latter is strong, the whole load is transferred to the fibre and the mechanical properties are high. If, on the other hand, the interfacial bond between matrix and fibres is low, a crack may initiate at and propagate along the fibre-matrix interface resulting in an premature failure. It Is therefore very important to enhance the bond between matrix and fibres.
In order to allow handling of the fibres, typically in a loom, and to enhance interfacial adhesion with the matrix they are embedded in, the fibres are coated with a sizing which composition depends on the nature of the reinforcing fibre to be sized and on the matrix the fibres are to be used with. Glass fibres are usually sized with a silane based composition since Si—O—Si covalent bonds can be formed between, on the one hand, the glass fibre surface and silanols obtained by hydrolysing of the alkoxysilanes of the sizing and, on the other hand, between adjacent silanol groups, thus forming a crosslinked structure at the surface of the glass fibres. This crosslinked structure seems to enhance the fibre resistance to corrosion, typically to hydrolysis. Adhesion of the sizing with the matrix is enhanced by the organic function of the silane coupling agent and by a film former, which nature depends on the matrix used. Sizing compositions usually comprise other additive such as a lubricant, antistatic agents, and the like. Numerous sizing compositions for glass fibres have been proposed in the art, as e.g., in WO2006007169, US2006204763, EP2053078, or WO2004110948 and are reviewed in E. P. Pluedemann, “Silane Coupling Agents”, Plenium Press (1982).
When monomeric alkoxysilanes or oligomeric polyalkoxysiloxanes with not more than ten units are usually used in sizing compositions for glass fibres, Park and Subramnian suggests in J. Adhesion Sci. Technol., 5 (6), 459 (1991) to use polymeric silanes in which pendant chains of siloxanes are attached through methylene chain spacers to a polyethyleneimine backbone. They suggest that a sawhorse structure is formed at the surface of the fibres, wherein two adjacent Si—O—Si groups between the silane polymer and the glass surface are linked by the polymeric backbone. They concluded that better results were obtained with dialkoxysilanes as side chains of the polymeric backbone, than with trialkoxysilanes. These conclusions were based on interfacial shear strength tests (IFSS) carried out on a single fibre embedded in a resin. Conclusions based on such test results cannot be generalized as such to actual composite materials, because in the latter the reinforcing fibres are packed into bundles of fibres which behave quite differently than individually embedded fibres.
One recurrent difficulty with continuous fibre reinforced composites is that the fibres are arranged in rather closely packed bundles of typically 800 to 8000 individual filaments (30 to 10,000 tex depending on the filament diameter of the order of 9 to 25 μm), which are difficult to penetrate in and percolate through by a liquid polymeric matrix, either a thermosetting resin precursor composition or a thermoplastic melt. The mechanical properties of a given composite (i.e., for a given matrix, fibre type, content and orientation) are dependent inter alia on the interfacial strength and the interfacial area. The former is enhanced by an appropriate sizing as discussed supra, and the latter depends on the ability of the liquid matrix to wet each individual fibre even within densely packed bundles. A poor wetting of the fibres by the matrix have particularly negative effects on the tensile strength in a direction normal to the fibre orientation (=90° tensile as defined in ISO 527/1), on shear strength (as best characterized with the so called short beam test (SBT) as defined in ISO 14130) and in particular in fatigue wherein failure at a stress substantially lower than the breaking point of a composite occurs after a number of load/unload cycles, which is a major source of failures in applications such as wind turbine, leafsprings, boat hulls and the like.
In spite of many developments in the field of sizing compositions for glass fibres, it remains a challenge to further optimize the load transfer from a polymeric matrix to the reinforcing fibres to approach the full mechanical potential of composite materials. The present invention proposes a solution towards this goal. This and other advantages of the present invention are presented in the following.